In the treatment of water to purify it for discharge into sewage systems, streams and the like, it is necessary or desirable to remove contaminants even when the contaminants are contained in very small concentrations such as a few hundred parts per million or less. In many situations industrial waste waters may contain small quantities of heavy metals such as copper, iron, lead, zinc, nickel, silver or gold with the concentration thereof varying from time to time over a substantial range. For example, the concentration of copper in an aqueous bath exiting a chrome plating system may vary from ten parts per million to thirty parts per million. Since, however, the concentration will peak at thirty parts per million only for brief periods, the prior art metal removal processes have not performed in an efficient manner when designed to function at the maximum concentration levels. Nevertheless, it is important that whatever process is used does not permit the heavy metals to be discharged even during the brief periods of maximum concentration.
It is known in the prior art that heavy metals can be precipitated out of aqueous solutions by placing the solution in a settling tank and adding insoluble starch-xanthate (ISX) to the solution. The ISX reacts with the heavy metal ions in solution to cause the heavy metals and the ISX to co-precipitate out of solution. The ISX may be prepared in any suitable manner such as by any of the methods described in U.S. Pat. Nos. 3,979,286 and 4,051,316. Japanese patent 46-39544 describes such a heavy metal removal process wherein the aqueous solution is treated with ISX. A prime disadvantage of the prior art processes of this type has been the need for holding tanks, settling basins or the like.
In my copending application, Ser. No. 961,238 filed Nov. 16, 1978, now abandoned there is disclosed a heavy metal removal process wherein a mixture of ISX and a filter-aid, such as diatomaceous earth, is coated on a filter element to form a filter cake through which the aqueous solution containing the heavy metals is passed. As the solution flows through the cake the heavy metal ions become attached to the active sites on the starch-xanthate and are thus removed from the solution. This process is better than the other prior art metal removal processes using ISX in that it may be used with a continuously flowing stream since no holding tanks, settling basins or the like are required. However, where the concentration of heavy metals varies, the amount of ISX deposited on the filter elements must be sufficient to react with all of the metal contained in the solution during the periods of maximum concentration. Consequently, this process necessitates the use of a greater amount of ISX than is required with the batch type processes of the prior art which operate on the average concentration of heavy metals in the solution being treated.
It is also known in the prior art that reactant material such as ion-exchange resins can be precoated on a filter element so as to react with a liquid solution passed through the resin precoat as the liquid flows through the filter element. U.S. Pat. No. 3,250,704 describes one such system which used a resin precoat in a water purfication process.
The precipitation processes known in the prior art and discussed above are not affected by momentary increases in concentration, peaks, as they are called, inasmuch as they are batch processes operating on the average concentration of the batch. However, where the water is purified as it continuously flows through the system as in the process described in my said copending application, it is absolutely necessary that the process functions to remove substantially all of the heavy metal during the peak concentration periods if the minimum discharge requirements are to be met. This can, of course, be achieved by the use of sufficient precoat volume to provide the necessary reaction during peak concentrations, but a considerably greater quantity of the insoluble starch-xanthate and filter-aid and a considerably greater filter area must be used than would be required to remove all of the heavy metal during the average or usual periods of concentration.